Attaching an Image creates a 'new image' file in the dropdown

Hi, I have encountered this issue with several websites that I have been working on recently.

Some of my sites have a sub-header image which sits in the middle content column, so for each top level navigation section there is a different picture. When I use the ImageFeild and save and publish, i go to the next page to add the image and a new file appears called 'new image' and when i am dealing with a fairly large site I get a huge long list of these building up thus slowing down the rate at which I can work (dropdown takes longer to load each time) and it makes it tedious scrolling past them all.

See attached image

Why is this file being created and how can I stop it? Do I just have to clear my cache every so often?